A cable modem network relies on the sharing of a common network media for the transfer of data between the Cable Modem Termination System (CMTS) and multiple Cable Modems (CM). Because this media is shared, both upstream and downstream network bandwidth must be conserved to maintain performance.
The Data Over Cable System Interface Specification (DOCSIS) introduced a concept of Payload Header Suppression (PHS). PHS allows a repetitive portion of certain payload headers to be suppressed by the transmitter and expanded by the receiver, thereby reducing the amount of data transmitted on the cable. This mechanism is implemented by software in first generation CM and CMTS devices, which produces significant processing overhead. This processing overhead serves to limit the number of CMs that can be supported by a single CMTS.
FIG. 1 illustrates a data packet being sent upstream with and without Payload Header Suppression. VoIP illustrates a normal RTP packet carried on an upstream channel. The beginning of the frame represents the physical layer overhead (FGPS) of FEC, guard time, preamble, and stuffing bytes. Stuffing bytes occur in the last code word and when mapping blocks to minislots. Next is the MAC layer overhead including the 6 byte MAC header with a 5 byte BPI Extended Header, the 14 byte Ethernet Header, and the 4 byte Ethernet CRC trailer. The VoIP payload uses a 20 byte IP header, an 8 byte UDP header, and a 12 byte RTP packet header. The voice payload is variable and depends upon the sample time and the compression algorithm used.
FIG. 1 also illustrates the same RTP packet with the Payload Header Suppression occurring in software. In the upstream, Payload Header Suppression begins with the first byte after the MAC Header Checksum. The 14 bytes Ethernet header, the 20 byte IP header, and the 8 byte UDP header have been suppressed, and a 2 byte PHS Extended Header element has been added, for a net reduction of 40 bytes. The suppressed bytes are designated “P” in FIG. 1. In this example of an established VoIP connection, these fields remain constant from packet to packet, and are otherwise redundant.
The detailed description that follows, describes a hardware mechanism which offloads a processing resource from the suppression and expansion duties. This mechanism reduces the processing resource requirements in both a CM and the CMTS, improving overall performance and allowing a larger number of CMs to be serviced by a single CMTS.